In electrophotographic machines of the transfer type, a moving photoconductor is charged to a relatively uniform level by a charging corona. The charged photoconductor is then imaged in order to produce a replica of an original on the photoconductor by variably discharging the charged photoconductor according to the image of the original. At this step in the process areas of the original which are white or light in background reflect or transmit a significant amount of light which, when reaching the photoconductor, discharge the photoconductor to an appropriate level. On the other hand, black or gray areas of the original document transmit or reflect much less light and therefore in these regions the photoconductor retains a significant charge. The next step in the process is to apply a developer to the image which may typically be a powder with a triboelectric charge of a polarity to be attracted to the undischarged portions of the photoconductor. Since the undischarged portions represent the black portions, a black toner is normally used in order to provide black copy, although in color copiers various other shades of toner are also used. After development the photoconductor moves to a position at which the developed image is transferred to a piece of copy paper or some other receiving medium. Transfer is effected through a corona generator which places a charge on the reverse side of the copy paper so as to attract the toner away from the photoconductor and onto the front side of the paper. After completing transfer the receiving medium passes through a fuser at which the toner is fused onto the copy paper or receiving medium and from there the copy paper passes out of the machine. The photoconductor, meanwhile, after the transfer is completed, continues to move to a cleaning station at which any remaining toner not transferred to the copy paper is cleaned from the photoconductor. The cleaned photoconductor then enters the charging station for a resumption of the copy cycle.
After transfer and prior to entering the cleaning station, it is necessary to neutralize the charge on the surface of the photoconductor by passing the photoconductor under a precleaning corona which is of opposite polarity to the charging corona. The photoconductor is also typically moved through the influence of an erase light in order to utilize light as a discharging medium for any remaining photoconductor charge. In that manner the cleaning station can operate to best advantage.
In the electrophotographic process a photoconductor may initially be charged either positive or negative depending generally upon the properties of the photoconductor chosen. Suppose that the charge on a particular photoconductor is negative. The result of imaging such a photoconductor is to leave a relatively low negative charge in all-white or lightly colored areas of the image and to leave relatively high negatively-charged areas in black or darkly colored areas of the image. Since it is desired to attract a toner to the highly negative areas the toner itself should take a positive charge. This charge is typically quite small since it is only that natural charge which is triboelectrically a part of the material used. Therefore, where the photoconductor is charged to a negative value the proper toner material will carry a positive triboelectric charge.
In magnetic brush developers, a magnetic material such as steel is ordinarily used as a carrier bead to move the toner from a sump area to the developing area. As a magnetic brush rotates, the steel carrier bead with the toner coated thereon is attracted to the rotating magnetic brush and rotates with the brush into the developing zone whereat the positive toner can be attracted to the negatively charged image. In order to ensure that the toner will be carried by the steel bead the steel is coated with tetrafluoroethylene, a synthetic resin which carries a natural triboelectric negative charge. Consequently, the positive toner is held by an electrical attraction to the negative tetrafluoroethylene-coated steel bead which is in turn magnetically attracted to the rotating developing brush. At the development area the triboelectric charge attraction between the positive toner and the negative coating is overcome by the more powerful negative charge on the photoconductor and in addition, due to the mechanical agitation at the developer area of the carrier and toner particles which tends to mechanically dislodge the toner from the carrier.
It has been found in systems utilizing tetrafluoroethylene-coated carrier particles that over a period of use small pieces of the coating are worn away from the bead and become a part of the developing process. Typically these tetrafluoroethylene-wear products are produced during the mechanical agitation at the development zone where the carrier beads are squeezed together as they pass through the restricted area between the surface of the magnetic brush and the surface of the photoconductor. These small wear particles retain their negative triboelectric charge and are attracted to the positive toner which in turn is attracted in great amount to the highly negatively charged photoconductor. The result often is that the small wear products leave the developing area on the surface of the photoconductor riding on the toner. The wear product, while quite small, may in some cases be considerably larger than the very small particles of toner and as a consequence it may create difficulties at the transfer station, causing imperfections in the reproduced copy. Note that since the tetrafluoroethylene carries a negative triboelectric charge it will not be attracted to the surface of the copy paper since the transfer corona is a negative corona intended to build up negative charge on the back side of the copy paper so that the positively charged toner is attracted from the photoconductor to the copy paper. That electrical system, however, repels the tetrafluoroethylene-wear product and therefore they continue to reside on the surface of the photoconductor after the photoconductor moves away from the transfer station. Hopefully, these particles will be cleaned off of the photoconductor at the cleaning station. If they are not successfully cleaned from the surface eventually they will be ground into the photoconductor and form a permanent coat called a "clear filming condition." Such a condition destroys the image reproducing qualities of the photoconductor and renders it unsuitable for continued use.
In addition to tetrafluoroethylene-wear products, other contaminants may come to reside on the surface of the photoconductor. For example, at the transfer station, a receiving medium is pressed against the photoconductor and a negative charge is placed on the backside of the paper. Dust may be present on the frontside of the paper and may be triboelectrically negative. As a result, that dust may be transferred to the photoconductor. Another contaminant is negatively-charged toner which, of course, does not transfer.
Having now described the problem and the genesis of contaminants in the electrophotographic process, the inventors herein have surmised that the preclean corona which is designed to neutralize the charge on the photoconductor may also be used to reverse the charge on the tetrafluoroethylene-wear products and other contaminants if the charge density produced by the corona generator is sufficiently high. By bombarding the negatively-charged particle with a sufficient amount of positive charge the particle can be made to assume a positive charge. As a consequence, this now positive contaminating particle can be successfully cleaned from the photoconductor at the cleaning station, together with the non-transferred, positively-charged toner. When a two-cycle process is in use with a combined cleaner/developer, on subsequent develop cycles the positively-charged contaminant may be attached to the negatively-charged image area in the manner in which toner is attracted. As a result, the contaminating particle rides to the transfer station where it is attracted to the copy paper and leaves the system on the surface of the copy paper. The inventors herein have recognized the value of high preclean corona currents in order to accomplish this objective and have carried this invention further to the point where they have discovered a definite relationship between the value of the transfer current and the proper setting for preclean corona current. The advantages of this invention are important for all electrophotographic machines, especially those which utilize tetrafluoroethylene-coated carrier and are doubly important in a two-cycle process machine in which the developing magnetic-brush roll is also used as the cleaning roll. This is due to the fact that if the negatively-charged contaminants are successfully cleaned from the photoconductor by the developer/cleaner, the particle enters the developer mix to become a part of that mix with the steel carrier beads and the positively-charged toner. As a result, the positively-charged toner is attracted to the negatively-charged contaminant as well as to the negatively-charged coated steel bead. If a sufficient amount of such contaminants exist in the developer mix, the result is to create poorly developed images. It is, therefore, extremely important in a two-cycle process with a combined developer/cleaner for the contaminants to be bombarded with a sufficiently high positive preclean corona current to change the charge on the contaminant from negative to positive. In that manner the contaminating particles will be carried out of the system on the copy paper as previously described and the developing station will continue to produce high quality copy.
It has been observed, however, that there is an upper limit to which the positive preclean corona current can be raised because of another problem called "toner filming" which results from too high corona currents. If a photoconductor becomes coated with toner the result is high background on reproduced copies and in general a lowering of the ability of the photoconductor to charge to its proper levels. This result occurs when the positive charges from the preclean corona build up to a significant extent on the outer surface of the toner remaining after transfer. Remember that the photoconductor was originally charged with a negative charge at the charge corona so that directly under the particle of toner lies a negative charge. With a high positive charge on the outer surface of the toner a significant gradient is established which tends to keep the toner in place on the photoconductor surface. Without that high charge present the attraction between the toner and the photoconductor is usually insufficient to cause a toner filming problem since the gradient can be overcome at the cleaning station where a higher valued negative charge is placed on the magnetic brush to attract the toner away from the photoconductor and back into the developing mix. However, should a high positive charge build up on the outer surface of the toner the negative bias at the cleaning station may be insufficient to clean the toner from the surface. Similarly, if a cleaning brush is used without electrical bias, the attraction between the toner and the photoconductor may be sufficient to prevent its being dislodged from the photoconductor by the cleaning brush. In any event it is clearly undesirable to apply too high a preclean corona current since the result is toner filming of the photoconductor surface.